Boston Marathon bomb suspect Tamerlan Tsarnaev took part in an earlier triple murder, according to a man subsequently shot dead while being questioned by authorities.
The revelation came in a court filing made by federal prosecutors in the case against his younger brother, surviving bombing suspect Dzhokhar Tsarnaev, 20.
According to the filing made on Monday, Ibragim Todashev told investigators Tamerlan Tsarnaev, 26, participated in a triple killing in Waltham, Massachusetts, on September 11 2011.
Three men were found in an apartment with their necks slit and their bodies reportedly covered with marijuana. One of the victims was a boxer and friend of Tamerlan Tsarnaev.
Todashev, a 27-year-old mixed martial arts fighter, was shot dead at his Orlando, Florida, home during a meeting with an FBI agent and two Massachusetts state troopers in May, authorities said, after turning violent while being questioned.
The filing is the prosecutors' attempt to block Dzhokhar Tsarnaev from obtaining certain information from authorities, including investigative documents associated with the Waltham murders.
"The government has already disclosed to Tsarnaev that, according to Todashev, Tamerlan Tsarnaev participated in the Waltham triple homicide," prosecutors wrote.
According to prosecutors, the ongoing investigation into the 2011 slayings is reason not to allow Dzhokhar Tsarnaev access to the documents he is seeking.
"Any benefit to Tsarnaev of knowing more about the precise 'nature and extent' of his brother's involvement does not outweigh the potential harm of exposing details of an ongoing investigation into an extremely serious crime, especially at this stage of the proceeding," prosecutors wrote.
Prosecutors also said Dzhokhar Tsarnaev was not entitled to the information because his brother's criminal history would be relevant, if at all, only at a possible future sentencing hearing.
Authorities say Dzhokhar and Tamerlan Tsarnaev, ethnic Chechens from Russia, planned and carried out the twin bombings near the finish of the marathon on April 15. Three people were killed and more than 260 injured.
Dzhokhar Tsarnaev faces 30 federal charges, including using a weapon of mass destruction and 16 other charges that carry the possibility of the death penalty.
Tamerlan Tsarnaev died in a gun battle with police as authorities closed in on the brothers several days after the bombings.